PROJECT SUMMARY Systemic lupus erythematosus (SLE) is a complex multi-system autoimmune disease characterized by the loss of tolerance to self-antigens and subsequent production of self-reactive antibodies. The most well-known and clinically-tested class of antibodies are the anti-nuclear antibodies (ANA), which include concurrent antibodies against a large family of ribonucleoproteins (anti-RNPs) such as the smith proteins (anti-Sm). We considered a possible cross-reactivity of the antibodies against shared antigen epitopes among the RNPs. Specifically, we noted a common type of posttranslational modification (PTM) known as symmetrical dimethylarginine (SDMA) prominently associated with these RNP molecules. To investigate whether these SDMA modifications contribute to PTM-promoted neoantigenic epitopes, we synthesized a large panel of test antigens from 76 proteins that carry SDMA marks. From probing 31 SLE patients, we detected strong antibody reactivity in about half of the patients towards SDMA-modified sequences, as opposed to the native epitopes with the same sequences. Meanwhile, selectivity for SDMA epitopes were not seen in heterogeneous healthy controls. The purpose of this ongoing study is to elucidate the potential involvement of arginine methylation in neoantigenic responses in lupus patients. The proposed study will comprehensively address the breadth of SDMA modifications in SLE autoantibody responses. Furthermore, we will use ex vivo and in vivo tools to determine the molecular mechanisms for anti-SDMA-mediated immune complexes. The working model that has its emphasis on chemically modified epitopes represents an important conceptual advancement in understanding autoimmune mechanisms. The positive outcomes of the study will also provide the molecular basis for the design of next-generation SLE tests with the incorporation of SDMA to the epitope residues.